On a dual boot system, how do I clone only the ubuntu partition & data to an external drive so the external drive is bootable

I have a dual boot Win10/Ubuntu 20.04 system. I rarely use the Windoze side, but have to on occasion. It has become unusable so I need to do a reinstall or reset or something.

But I don't trust Windows to handle the reset and not futz up my Ubuntu partition. So I'd like to clone the Ubuntu partition to an external drive and be able to boot from it just in case Windows screws up the reset and trashes my ubuntu partition.

Is that possible?

I've spent hours looking and find contradictory advice. Some posts say use ddrescue but not to specify partition numbers, i.e., /dev/sda and not /dev/sdaX, which, of course would clone the Windows partition also and not simply the ubuntu one. Others say specifying a partition is fine, but don't explain how to make the cloned external bootable if you do that.

Any suggestions?

Thanks.